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19 Sep 2014. NUS professors have proposed a scheme to implement quantum computation with surface-state electrons by rapid population passages.

Computers based on the laws of quantum mechanics are much more powerful than present computers. It has been shown that quantum computation can overcome the limits of classical computing, for instance, in the factoring of large numbers and searching databases. Quantum computing is based on logic gates and the central task is to coherently control the transitions between the quantum logic states for implementing various gate operations. A team led by Prof OH Choo Hiap from Centre for Quantum Technologies and Department of Physics in NUS and Southwest Jiaotong University (China) has proposed a scheme to implement logic gates by using population passage technique, and it may be realized experimentally by electrons on liquid helium.

The team’s scheme is based on control of surface states of electrons floating on liquid helium. It can be utilized to implement single andtwo-qubit gates by using the so-called population passage technique, wherein the durations of the applied pulses are not required to be precisely designed (see Figure). In particular, the deterministic population transfers between the selected quantum states can be achieved by applying either adiabatic or non-adiabatic pulses to thequbit(s). With the proposed non-adiabatic passage technique, the Bell state may be deterministically generated.

With the advantage of long decoherence time and scalability, quantum manipulations of surface-state electrons on liquid helium have attracted much attention recently. Making use of the evolution-time insensitivity, they propose the adiabatic and non-adiabatic population passages to implement quantum computation with such electrons. Until now, almost all proposals are based on the usual Rabi oscillation scheme, wherein the applied pulses need to be exactly designed to implement the desired quantum gate operations. But in this new scheme, the exact design is not required. Furthermore the desired population transfers between the logic states can be deterministically realized.

(Left) Two electrons on liquid Helium controlled by the micro-electrodes. Population passages for single-qubit gate is controlled by two fields and and two-qubit gate is realized by coupling the single qubit with Coulomb interaction. (Right) Population evolution of the single-qubit state along different passages. Obviously, population transfer along the non-adiabatic passage is faster than that along the adiabatic passage, the same conclusion is in accord with the two-qubit operation. (Picture credit: OH Choo Hiap)